The present invention relates to a method of hardening soft ground and, more particularly, a method of hardening soft soil such as clay, silt or the like by mixing it with a cement-based hardener.
It is widely known that Japanese islands consist of soft ground most predominantly among the world. Recently, as urban development proceeds in Japan, a lack of land has come to present a serious problem since habitable land areas are extremely limited in Japan. For this reason, unused soft ground is improved or hardened so that structures may be constructed thereon.
As a matter of fact, a wide variety of methods have heretofore been proposed to improve such ground.
However, to cope with a general demand for shortening the total construction period and increasing the size and weight of structures, the period to be involved in the hardening performance is required to be shortened and the strength of soft ground is also required to be more securely improved. Moreover, it is strongly desired that environmental destruction and pollution through such performance be prevented.
In view of the foregoing, there has recently been developed a method of improving soft ground with the aid of a cement-based hardener, namely, such a method of hardening the soft ground by mixing a cement-based hardener with soft soil.
In this method, mixing propellers of a ground improvement machine are penetrated into soft ground so as to mix a cement-based hardener with soft soil.
In such method of hardening soft ground, the following measures have heretofore been taken according to performance and soft soil conditions:
(1) improved soil piles wherein the soft soil and a cement-based hardener are mixed by the ground improvement machine are successively cemented together to thereby harden the overall soft ground; and
(2) deep walls which reach the bearing stratum and short walls which do not reach it are jointly formed one after the other to thereby harden the soft ground.
In the measure (1), upon the extraction of the ground improvement machine from the soft ground, slurry consisting of a mixture of the cement-based hardener and water is supplied from a position located above the mixing propellers while the soft soil and the hardener are mixed upon rotation of the mixing propellers so as to form mixed layer of the soft soil and the cement-based hardener. The soft soil is thus hardened on account of the hardening effect of the cement.
More concretely, as shown in FIG. 1, a ground improvement machine 1 is provided with mixing propellers 3 mounted to the lower part of shafts 2, said machine 1 being moved upward and downward by means of a driving means 4 whereby said mixing propellers are rotated.
To begin with, for example, the machine 1 is penetrated into a soft ground 5 until it reaches bearing stratum G. The machine 1 is extracted in the direction of an arrow A whereby the mixing propellers 3 are rotated therewith. At the same time, a cement-based hardener in a slurry state is supplied from a position located above uppermost mixing propellers 3A, for example, a position adjacent to the front end of an arrow B while the soft soil and the hardener are mixed. Furthermore, while the machine is extracted, improved soil pile consisting of mixed layer 6 of the soft soil and the hardener is formed. Then, another improved soil pile is jointly formed with this soil pile in the same manner as above and, thereafter, the same process is repeatedly carried out in order to harden the overall soft ground.
In such measure, however, as shown in FIG. 2, an outlet 7 for supplying the cement-based hardening slurry is fixedly provided above the mixing propellers 3, so that the hardener is supplied to only one fixed point in the soft ground. Thus, the mixture of the hardening slurry and soft soil is dependent only upon mechanical mixing of the mixing propellers 3.
Thus, the distance the hardening slurry is to be moved from the outlet 7 by the propellers 3 is relatively great. For this reason, the hardening slurry and soft soil are not readily evenly mixed and therefore, a considerable amount of time is necessary for the even mixture thereof.
Further, as stated above, since the location at which the hardening slurry is supplied is fixed, the area where the hardening slurry and soft soil may be mixed is limited even if the diameter of the mixing propellers 3 is increased under existing performing conditions, although it may be dependent upon the penetrating or extracting velocity of the machine 1 and the rotational number of the mixing propellers 3.
It should be mentioned that normally, the mixing propellers 3 are approximately one meter (1 m) in diameter at the most. There exists a problem, therefore, that the machine must repeatedly be shifted a number of times so as to improve a wide range of ground.
Another problem to be faced when the new improved soil pile is cemented to existing improved soil pile is such that some unimproved ground may remain therebetween unless the route taken upon the penetration of the machine and that taken upon the extraction thereof are completely identical or the machine is positioned closer to existing hardened ground side. As a matter of fact, as shown in FIG. 1, upon the extraction of the machine 1 the front end of the machine 1 tends to move outside, namely in the direction of an arrow C where mixing resistance is relatively small. Consequently, unimproved ground 9 remains between existing hardened ground 8 and mixed layer 6 newly formed by the hardening slurry and soft soil and therefore, hardened ground may not successively be formed.
In order to closely joint the existing improved ground 8 with the newly formed mixed layer 6, as shown in FIG. 3, the front end of the machine 1 must be pressed onto the existing improved ground 8 upon the extraction of the machine 1 thereby to cut the existing improved ground 8.
Further, in the event that the hardening process of the existing improved ground 8 has already commenced, such cutting is difficult to be carried out.
Especially, in the event that the hardening performance is discontinued due to bad weather or mechanical trouble, the soft soil mixed with the cement-based hardener becomes completely hardened. Hence, it may be impossible to joint the existing improved ground with a newly formed layer.
On the other hand, the measure (2) is such that as shown in FIG. 4, deep walls 11 and short walls 10 are formed one after the other thereby to harden the overall soft ground. As shown in FIG. 5, for example, when the deep walls 11 are jointly formed with the short walls 10, the deep walls 11 are supported on the bearing stratum G whereas the short walls 10 are not supported thereon although they are in abutment with the soft ground 5. Furthermore, as the ground improvement machine cuts the short walls when it is penetrated or extracted so as to facilitate the better cementing between the deep walls 11 and short walls 10, the short walls are lowered or raised by the mixing propellers upon the penetration or extraction of the ground improvement machine.
Especially, in a conventional method as mentioned above, the cement-based hardening slurry is supplied upon the extraction of the ground improvement machine. Accordingly, as shown in FIG. 6, unless the route taken upon the penetration of the machine (unbroken line) and that taken upon the extraction thereof (broken line) is identical, unhardened soil X may be left. There is a problem, therefore, that the deep walls 11 and short walls 10 may not sufficiently be cemented together.
In order to overcome this problem by a conventional method, the route taken upon the penetration of the machine and that taken upon the extraction thereof are required to be identical. However, it should be mentioned that this is not actually possible.